FIELD OF THE INVENTION
The present invention relates to a plating method for a nickel-titanium alloy member, and more specifically, to a method for forming a plating layer on the surface of the nickel-titanium alloy member with high adhesion. oxide film exists on the surface of each nickel-titanium alloy member from the beginning, however, the members cannot be easily brazed or soldered in this state.
In many cases, therefore, screwing, riveting, caulking, and other mechanical methods are used to connect the nickel-titanium alloy members to one another.
If any of these method is employed, however, the appearance of spectacle frames may possibly be marred, for example. In the case of an electrical component, moreover, electrical connection failure is liable to occur at the junctions.
Furthermore, the nickel-titanium alloy members may be also connected by soldering or brazing after they are plated with nickel or copper.
In this case, the nickel-titanium alloy members are dipped in hydrochloric acid, a liquid mixture of fluoric acid and nitric acid, a liquid mixture of hydrochloric acid and nitric acid, or a liquid mixture of hydrochloric acid, sulfuric acid and nitric acid, for pickling, whereby the oxide film on the surface of each member is removed by dissolution as a pretreatment, and the member surface is then plated with nickel or copper. The pretreatment serves to improve the adhesion between the surface of each nickel-titanium alloy member and the plating layer formed thereon.
Despite the pretreatment, however, the adhesion between the plating layer and the surface of each nickel-titanium alloy member cannot always be satisfactory, and the formed plating layer may often be cracked or separated from the member surface. When the plated nickel-titanium alloy members are bonded together by soldering or brazing, moreover, the bonding strength is low, and the electrical connection is unstable.
Supposedly, these problems are attributable to the following reason.
Conventionally, the solution used for the pickling contains nitric acid, because the nitric acid contained serves to enhance the capacity of removing the oxide film existing from the outset. Since nitric acid has an oxidative effect, however, a new oxide film is formed on the surface of the nickel-titanium alloy member. Although the newly formed oxide film is thinner than the oxide film having been existing on the surface of the member from the beginning, it adversely affects the adhesion of the plating layer formed, all the same. Where hydrofluoric acid is contained in the solution, it is not essential to mix nitric acid as mentioned above. In this case, however, difficulties arise in the disposal of waste liquid containing hydrofluoric acid.